Adaptive discovery and correction of phase alignment errors in monopulse antenna systems

ABSTRACT

A mainlobe detection process can include a number of tests that are performed to define when the monopulse antenna system will transition from open loop scanning to closed loop scanning and then to tracking. A hybrid tracking technique is also provided which adaptively discovers and corrects for phase alignment error. Magnitude-only tracking can be performed initially to locate the nulls in the azimuth and elevation ratios and to identify the magnitudes of these ratios at these nulls. Phase tracking can be then performed. During phase tracking, phase corrections can be repeatedly applied to the azimuth and elevation difference channels to correct any phase error that may exist. During this process, the magnitudes of the ratios can be used to determine how the phase corrections should be adjusted. Once the hybrid tracking process is complete, the monopulse antenna system is properly phase-aligned and phase tracking will be correctly employed.

CROSS-REFERENCE TO RELATED APPLICATIONS

N/A

BACKGROUND

A monopulse antenna system is commonly used to implement radar trackingor to track intentional radiators. As its name implies, a monopulseantenna system employs a single pulse to identify the presence of anobject in the field of view. This is possible due to the use of multipleantennas which can detect angle information from the arriving signal.

FIG. 1 illustrates an example block diagram of a typical dual-axismonopulse antenna system. As shown, a dual-axis monopulse antenna systemmay include four individual antennas (A, B, C, and D) each of which aredriven by the same arriving signal. If an object is in the field of viewof the monopulse antenna system, each individual antenna will receivethe reflected (or transmitted) signal. These received signals (which arereferred to as A, B, C, and D respectively) are then fed to a comparatornetwork. Although not shown, some intermediate processing may beperformed on the received signals prior to inputting them to thecomparator network.

Due primarily to the slight differences in the positions/orientations ofthe individual antennas, the characteristics of the received signalswill vary. The comparator network can detect these variations to allowthe relative location of the object with respect to the boresight axisto be determined. In particular, the comparator network can generatethree tracking channels: (1) a sum (Σ) of the four received signals; (2)an azimuth difference (Δ_(az)); and (3) an elevation difference(Δ_(el)). As one of skill in the art would understand how these trackingchannels can be employed to identify and track the position of anobject, no further description will be provided.

As with most antennas, monopulse antennas produce a mainlobe (or mainbeam) and various sidelobes. In many situations, it will be possible todetect the presence of an object (or intentional radiator) whenever theobject is positioned within the mainlobe or within one of the sidelobesdue to the relatively high gain of some sidelobes. Therefore, even ifthe monopulse antenna is not pointed directly at the object, it maystill receive a strong enough signal to detect the object's presence.However, if the object is within a sidelobe, and if the comparatornetwork detects a sum channel peak or a difference channel null, theantenna will incorrectly assume that it is pointing directly at theobject.

In typical monopulse antenna systems, an open-loop GPS and navigationdata backbone is employed to perform coarse tracking. In other words,GPS data of the object to be tracked is supplied to the monopulseantenna system to allow the monopulse antenna system to initially pointthe antenna in the general direction of the object. Using GPS data inthis way also requires that the antenna be physically oriented withrespect to true north which can be a tedious process.

Additionally, as part of this tracking system, a modem lock signal willtypically be employed as an indicator to the system that tracking isoccurring. At sufficiently large target ranges, because the mainlobegain is higher than the sidelobe gains, a SNR sufficient to establishthe modem lock should only exist when the object is within the antenna'smainlobe. However, the fact that a modem lock can be established doesnot necessarily imply that the object is within the antenna's mainlobe.In many situations, an adequate SNR for establishing a modem lock mayexist even though the object is positioned within the antenna'ssidelobe. In such situations, the monopulse antenna system will trackthe object using a sidelobe when the desired outcome is to track withinthe mainlobe. This increases the risk of dropping the link due tomarginal signal-to-noise ratio performance as the target moves away.Accordingly, a modem lock is a poor indicator of mainlobe tracking.

BRIEF SUMMARY

The present invention extends to techniques for implementing a mainlobedetection process in a monopulse antenna system. The mainlobe detectionprocess can include a number of tests that are performed to define whenthe monopulse antenna system will transition from open loop scanning toclosed loop scanning and then to tracking. Once tracking has commencedand all tests have passed, it ensures that the target is within themainlobe of the monopulse antenna. The present invention also extends toa hybrid tracking technique in which a magnitude-only tracking method isperformed prior to phase tracking to allow phase tracking to facilitatethe auto-correction of phase-alignment errors between the monopulseantenna system and the monopulse antenna feed comparator network.Magnitude-only tracking can be performed initially to locate thedifference channel nulls and to identify the average magnitudes of thedifference-to-sum channel tracking ratios at these nulls. Phase trackingcan then be performed. During phase tracking, phase corrections can berepeatedly applied to the azimuth and elevation difference channels toattempt to offset any phase error that may exist. During this process,the magnitudes of the tracking ratios near the difference channel nullsare used to determine how the phase corrections should be adjusted.

In another embodiment, the present invention is implemented as a methodfor performing tracking in a monopulse antenna system that providestracking in the azimuth axis or the elevation axis or in both theazimuth and elevation axes. While magnitude-only tracking is beingperformed, a first azimuth magnitude parameter that represents amagnitude of the azimuth ratio over a number of magnitude steeringiterations and/or a first elevation magnitude parameter that representsa magnitude of the elevation ratio over the number of magnitude steeringiterations can be stored. Additionally, an azimuth direction indicatorthat represents a phase of the azimuth ratio over the number ofmagnitude steering iterations and/or an elevation direction indicatorthat represents a phase of the elevation ratio over the number ofmagnitude steering iterations can also be stored. After magnitudetracking has been performed for the number of magnitude steeringiterations, the azimuth direction indicator and/or the elevationdirection indicator can be compared to a defined threshold. Upondetermining that the azimuth direction indicator and/or the elevationdirection/indicator is below the defined threshold, phase tracking canbe commenced.

In another embodiment, the present invention is implemented as amonopulse antenna system comprising: a monopulse antenna comprising anumber of monopulse antenna elements; a comparator network thatgenerates a sum channel, an azimuth difference channel, and an elevationdifference channel from a signal received at the monopulse antenna; anda monopulse detector assembly that receives the sum channel, the azimuthdifference channel, and the elevation difference channel from thecomparator network and generates an azimuth ratio and an elevation ratiofrom the channels. The monopulse detector assembly is configured toperform hybrid tracking in which magnitude-only tracking is performedprior to phase tracking.

In another embodiment, the present invention is implemented as a methodfor detecting the mainlobe in a monopulse antenna system and thentracking on the mainlobe. During open loop scanning, an initialpower-level test is performed to identify when a lobe has been located.In response to the initial power-level test passing, closed loopscanning is commenced. During closed loop scanning, a track-lock test isperformed to identify when the mainlobe has been located. In response tothe track-lock test passing, hybrid tracking is commenced in whichmagnitude-only tracking is initially performed and then phase trackingis performed.

This summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 provides a block diagram of a typical monopulse antenna system;

FIG. 2 illustrates a block diagram of a monopulse antenna system that isconfigured in accordance with one or more embodiments of the presentinvention;

FIGS. 3A and 3B illustrate a flow chart of a mainlobe detection process;

FIG. 4 provides a state diagram of the mainlobe detection process;

FIGS. 5A-5C provide various charts representing how the mainlobedetection process can detect the mainlobe and the process results uniqueto the mainlobe;

FIGS. 6A-6C provide various charts depicting how an antenna feed phasealignment error may exist;

FIGS. 7A-7B illustrate a state diagram of a hybrid tracking process;

FIG. 8 illustrates a block diagram of a monopulse antenna system that isconfigured to implement the hybrid tracking process in accordance withone or more embodiments of the present invention; and

FIGS. 9A and 9B illustrate an example of how a phase shifter can beupdated to apply a different phase correction during the hybrid trackingprocess.

DETAILED DESCRIPTION

MDA 203 can be configured to implement the techniques of the presentinvention to ensure that monopulse antenna 201 is oriented such that theobject to be tracked will be positioned within the mainlobe rather thana sidelobe. In other words, MDA 203 can generate steering signals tocontrol the orientation of monopulse antenna 201 in accordance with thetechniques of the present invention as will be described below.Monopulse antenna system 200 may also typically include a processor 204which can interface with MDA 203 to provide control signals to and toreceive state and tracking information from MDA 203. Also, processor 204can be configured to interface with monopulse antenna 201 for thepurpose of providing steering signals. Although not shown, monopulseantenna system 200 may include a steering component with which MDA 203and processor 204 interface for purposes of steering monopulse antenna201.

By way of overview, the present invention utilizes a series of testsduring the steering of the monopulse antenna to ensure that tracking isonly performed on the mainlobe. Initially, the monopulse antenna can besteered in an open loop scan mode. Open loop refers to the fact that themonopulse antenna is steered independently of the RF signal it receives.For example, while in open loop scan mode, the monopulse antenna can besteered relatively quickly along a spiral or zig zag pattern in anattempt to locate an object. Processor 204 can provide the steeringsignals to monopulse antenna 201 when in the open loop scan mode. Incontrast, closed loop scanning refers to steering the antenna based onthe azimuth and elevation tracking errors that are produced from thechannel measurements. Therefore, closed-loop tracking often involvesrelatively slow/small movements in comparison to open loop scanning.Because closed loop scanning is based on the tracking channels, MDA 203can be tasked with steering monopulse antenna 201 during closed loopscanning. In either case, the monopulse antenna can be steered eithermechanically (i.e., by physically moving the monopulse antenna) orelectrically (e.g., by using phased array techniques).

Continuing the overview, while processor 204 steers monopulse antenna201 in open loop scanning, MDA 203 can process the three trackingchannels to determine whether an object appears in the field of view ofmonopulse antenna 201 (e.g., based on a comparison of the sum channelpower to the difference channel powers). Once MDA 203 detects that anobject is present within the field of view (defined as the initialpower-level test conditions), control of the steering can be turned overto MDA 203 to allow MDA 203 to determine whether the object is locatedwithin the mainlobe of monopulse antenna 201. If MDA 203 determines thatthe object is within the mainlobe, it declares track lock and continuesto track the object. However, if MDA 203 determines that the object iswithin a sidelobe, it can pass steering control back to processor 204 torecommence open loop scanning. This process can be repeated as necessaryuntil the monopulse antenna 201 is steered towards the object such thattracking can occur on the mainlobe.

FIGS. 3A and 3B provide a flowchart of this mainlobe detection process.As indicated above, MDA 203 is primarily tasked with performing thesteps of this process. Therefore, FIGS. 3A and 3B generally representthe steps performed by MDA 203. However, processor 204 can also beinvolved in providing various control inputs and in monitoring the stateof the process. The interactions that may occur between MDA 203 andprocessor 204 are described more fully below with respect to the statediagram of FIG. 4.

The mainlobe detection process can commence when open loop scanning isenabled. For example, processor 204 (or more specifically, softwareexecuting on processor 204) could instruct MDA 203 to commence themainlobe detection process. When open loop scanning is commenced, aninitial power-level test can be iteratively performed by MDA 203. Asshown, this initial power-level test can include (1) determining whetherthe sum channel power is greater than each of the difference channelpowers. In some embodiments, this initial power-level test may alsoinclude (2) determining whether the azimuth ratio and the elevationratio are each less than an initial power-level threshold.

When an object is present within the field of view of monopulse antenna201, the sum channel power will exceed both the azimuth channel powerand the elevation channel power. Therefore, when MDA 203 detects thatthis condition is met, it can transition into a closed loop scanningmode. The azimuth and elevation ratios represent the ratio of thedifference between the sum channel power and the azimuth and elevationchannel powers respectively and can therefore serve to define athreshold for when the initial power-level condition will cause atransition into the closed loop scanning mode. In other words, thissecond condition of the initial power-level test can prevent closed loopscanning from being performed when the sum channel power only slightlyexceeds the difference channel powers.

This initial power-level threshold (as well as the track-lock thresholddescribed below) can be a configurable parameter so that the“sensitivity” of the process can be fine-tuned for a given environment.For example, in environments when multipath reflections may occur (e.g.,when monopulse antenna system 200 is positioned above the object to betracked and will therefore receive reflections from the earth'ssurface), a higher value for the initial power-level threshold may bedesirable.

As mentioned above, the transition from open loop scanning to closedloop scanning involves allowing MDA 203 to control the steering ofmonopulse antenna 201 based on the RF signal received by the antenna(i.e., based on the values of the three channels). The fact that theinitial power-level test has succeeded indicates that an object ispresent within a lobe of monopulse antenna 201. However, at this point,it is still unknown whether the object is within the mainlobe or withinone of the sidelobes.

FIG. 5A provides a plot identifying the areas where the sum channel gainexceeds both difference channel gains. This plot can be interpreted asthe composite (including sum, azimuth, and elevation channels) gainprofile that would be seen when looking directly at monopulse antenna201. Accordingly, the mainlobe is centered at 0° azimuth and 0°elevation and a number of sidelobes are positioned around the mainlobeas indicated. It is noted that this plot is simplified for illustrativepurposes to remove various weaker sidelobes. In this example, if anobject was positioned within the mainlobe or any of the sidelobes, thereceived signal would exhibit characteristics that would cause theinitial power-level test to pass. In other words, even when the objectis in one of the labeled sidelobes, the sum channel power would stillexceed both difference channel powers. Both the azimuth and elevationratios should also be below the initial power-level threshold when theobject is in one of the labeled sidelobes. Accordingly, the initialpower-level test functions as a “power test” for identifying when anobject is located within a lobe.

To ensure that MDA 203 does not commence tracking the object when theobject is within a sidelobe, MDA 203 can be configured to perform twoadditional tests. Initially, upon commencing closed loop scanning, MDA203 can perform a track-lock test. As with the initial power-level test,the track-lock test can involve: (1) ensuring that the sum channel powerremains greater than both the difference channel powers; and (2)determining whether the azimuth ratio and the elevation ratio are eachless than a track-lock threshold. This track-lock threshold can besubstantially less than the initial power-level threshold in order toincrease confidence that the object is within the mainlobe. By way ofexample only, the initial power-level threshold may be 50% and thetrack-lock threshold may be 5%. However, as indicated above, thesethresholds can be configurable to fine-tune the system for a particularenvironment.

This track-lock test is iteratively performed while MDA 203 steersmonopulse antenna 201 based on the received values of the trackingchannels. In other words, MDA 203 makes small adjustments to theorientation of monopulse antenna 201 to attempt to center the objectwithin the lobe. As this centering is performed, the sum channel powerrelative to the difference channel powers should increase (i.e., MDA 203will steer monopulse antenna 201 to minimize the azimuth and elevationratios).

Each time the track-lock test is performed and fails, the timeoutparameter can be incremented. The timeout parameter is used to set atest parameter to determine if the system will pass the test in asuitable amount of time. When testing near the mainlobe, the test willalways pass under a certain time period. However, for locations otherthan the mainlobe, the test may never pass, but closed-loop trackingwould continue were it not for a timeout period. If the timeout valuereaches a particular value (e.g., 100), it can be assumed that theobject is within a sidelobe, or certainly not in the mainlobe. Morespecifically, if the object is within a sidelobe, there may be noorientation within this sidelobe that will cause the track-lock test topass. As a result, MDA 203 can pass steering control back to processor204 to recommence the open loop scanning process to attempt to locatethe mainlobe. In contrast, if the track-lock test passes, it impliesthat the sum channel power greatly exceeds the difference channelpowers, and, as such, the object may likely be within the mainlobe andtracking can be commenced.

FIG. 5B provides a plot illustrating the area where both the azimuth andelevation ratios are below the track-lock threshold (e.g., below 5%). Asshown, in this case, the conditions of the track-lock test are met onlywithin a small portion (the + shaped portion) of the mainlobe.Accordingly, if during open loop scanning, the object comes within oneof the sidelobes and passes the initial power-level test, closed loopscanning will commence within that sidelobe. Because the ratios requiredby the track-lock test do not exist within the sidelobe, the track-locktest will eventually time out causing open loop scanning to be resumed.In contrast, if during open loop scanning, the object comes within themainlobe, the subsequent closed loop scanning will cause monopulseantenna 201 to eventually be oriented directly towards the object (i.e.,oriented so that the object is within the + shaped region of FIG. 5B)thereby causing the track-lock test to succeed.

The initial power-level and track-lock tests can be performed veryquickly such that a number of closed loop scanning intervals can beperformed in a relatively short amount of time. Therefore, even if thetrack-lock test is performed on a number of sidelobes prior to reachingthe mainlobe, mainlobe tracking can ultimately be achieved in areasonable amount of time. The values of the initial power-level andtrack-lock thresholds can be set to control how quickly this acquisitionmay occur.

In some embodiments, once the initial power-level and track-lock testshave passed, MDA 203 can commence tracking the object using the currentvalues of the azimuth and elevation ratios. In some embodiments, thistracking can be performed in a “magnitude-only mode” in which themagnitudes alone of the azimuth and elevation ratios (or tracking error)are used to steer monopulse antenna 201 (i.e., tracking is performedindependently of the phase). During this tracking, a mainlobe-check testcan be continuously performed to ensure that the object remains withinthe mainlobe. As shown in FIG. 3B, this mainlobe-check test can be thesame as the initial power-level condition of the initial power-leveltest, namely, whether the sum channel power remains greater than boththe difference channel powers. If, during tracking, the object movesoutside of the mainlobe, this mainlobe-check test will fail therebycausing MDA 203 to return steering control back to processor 204 whichmay resume the open loop scanning process in an attempt to again orientthe mainlobe towards the object.

Even with a low value for the track-lock threshold, there may still besome very infrequent scenarios where the track-lock test will pass whenthe object is located within the sidelobe (e.g., when the target isrelatively close to monopulse antenna 201 and strong receive signals arepresent). The discriminator for these circumstances is built-in to theinherent phase response of the antenna feed comparator network, and canonly be exploited when in phase tracking mode. In other words, whenclosed-loop tracking attempts to pass the track-lock test, MDA 203 couldbe commanded to be run in either magnitude-only mode or phase mode fortracking. Only in phase tracking mode can the MDA discriminate againstsituations where the azimuth or elevation ratio magnitudes are less thanthe track-lock threshold when the target is actually on a sidelobe. Asan overview, this phase tracking can employ the phase of the azimuth andelevation tracking ratios (as represented by the sign of the azimuth andelevation ratios) to steer monopulse antenna 201 towards a center of themainlobe. Due to the differences between the phase pattern within themainlobe and the phase patterns within the sidelobes, when an object isbeing tracked within a sidelobe, this phase tracking will causemonopulse antenna 201 to be steered away from the center of thesidelobe. This will ultimately cause the mainlobe check test to failthereby causing open loop scanning to be resumed.

FIG. 5C provides a plot illustrating the phase pattern that existswithin the mainlobe and the various sidelobes that are represented inFIG. 5A. As is known, each of the A, B, C, and D signals produced by amonopulse antenna will have a phase. In theory, if the object is locateddirectly in the center of the mainlobe and the phase response of themonopulse antenna system is properly aligned, each of these signals willhave the same phase. However, if the object's location is offset fromthis “zero phase” position, and phase is taken into account, the azimuthand elevation difference ratios can be either positive or negative. Forexample, if the object is located at 0.5 degrees azimuth, the phasedifferences in the A, B, C, and D signals may cause the azimuthdifference ratio to be positive, while if the object is located at −0.5degrees azimuth, the phase differences in the A, B, C, and D signals maycause the azimuth difference ratio to be negative. A similar transitionin the phase may occur along the elevation axis.

MDA 203 can employ these azimuth and elevation phase transitions duringtracking. More particularly, MDA 203 can steer monopulse antenna 201based on the sign of the azimuth and elevation ratios (or equally basedon the sign of the azimuth and elevation difference channels). Withreference to FIG. 5C, if the tracked object is located in either of theright-sided quadrants (relative to the azimuth angle 0° reference), theazimuth ratio may have a non-zero positive value, and if the trackedobject is located in either of the left-sided quadrants (relative to theazimuth angle 0° reference), the azimuth ratio may have a non-zeronegative value. Similarly, if the tracked object is located in either ofthe top quadrants (relative to the elevation angle 0° reference), theelevation ratio may have a non-zero positive value, and if the trackedobject is located in either of the bottom quadrants (relative to theelevation angle 0° reference), the elevation ratio may have a non-zeronegative value. MDA 203 can be configured to steer monopulse antenna 201based on the signs of these ratios. The tables below provide one exampleof how MDA 203 can be configured to steer monopulse antenna 201 whenperforming phase tracking. It is noted that the sign of the ratios willdepend on how the azimuth and elevation difference channels are produced(e.g., whether the azimuth difference channel is produced as A+C−B−D orB+D−A−C) and therefore the steering directions are relative to how thedifference channels are produced.

Sign of Direction Sign of Direction Azimuth Ratio to Steer ElevationRatio to Steer + Right + Up − Left − Down

As can be seen, based on these rules during phase tracking, MDA 203 willcontinuously steer monopulse antenna 201 to cause the object to belocated at the phase transition point in each axis (i.e., where theazimuth and elevation ratios approach zero) which, in theory, shouldexist at the point where the ratios are minimized (i.e., at the pointwhere the difference between the sum channel gain and the differencechannel gains is maximized). Due to this tracking, monopulse antenna 201will remain oriented properly towards the object such that themainlobe-check test will repeatedly succeed.

Turning again to FIG. 5C, it is noted that the phase pattern within thesidelobes is not the same as the phase pattern within the mainlobe. Forthis reason, if phase tracking is being performed on a sidelobe, MDA 203will cause monopulse antenna 201 to steer away from the object. Thiswould force a failure during the mainlobe check test. For example, ifthe object is located within the top left (or white) quadrant of theupper sidelobe, the elevation and azimuth ratios will both have apositive sign. Based on the rules above, this will cause MDA 203 tosteer monopulse antenna 201 in an upward and rightward direction. Inother words, the signs of the ratios will cause MDA 203 to believe theobject is above and to the right of the center point of the lobe when infact the object is above and to the left of the center point. As aresult, after MDA 203 steers monopulse antenna 201, the object willeventually be located outside the upper sidelobe where the sum channelpower will not be greater than the difference channel powers therebycausing the mainlobe check test to fail. In this way, MDA 203 employsphase to cause the mainlobe check test to fail when tracking isperformed on a sidelobe thereby causing open loop scanning to be resumedin order to locate the mainlobe.

In some embodiments, the track-lock test may include an additionalcondition which monitors the variation in the sum channel power. When onthe mainlobe, there should, in theory, be very little variation in thesum channel power across readings. In contrast, when on a sidelobe, thesum channel power may vary substantially. MDA 203 can therefore recordthe sum channel power level at each sampling and use these recordedlevels to verify whether the power level has varied in excess of somethreshold (e.g., whether the sum channel power level has varied by morethan 2 dB over a series of ten samples). If the sum channel power variesin excess of this threshold, the track-lock test can fail therebycausing open loop scanning to be resumed. This variance threshold can bea configurable parameter as with the other thresholds. For example, inmultipath conditions or for a fast-moving target, the sum channel powermay vary significantly by comparison when on the mainlobe. In suchcases, the variance threshold can be set to a higher level to preventthe track-lock test from incorrectly failing. A primary benefit ofemploying the variance threshold in the track-lock condition is that itmay cause the track-lock test to fail before the timeout has beenreached. In this way, less time will be spent doing closed loop scanningwithin a sidelobe.

FIG. 4 illustrates a state diagram for the above described process andrepresents how MDA 203 and processor 204 can interact. A number ofcontrol and state parameters can be employed to enable processor 204 tomonitor and control MDA 203 during the process. These parameters includea ScanFlag parameter which when set will enable the scanning process, aScanMode parameter which defines whether open or closed loop scanningwill be performed, a Timeout parameter which tracks the number ofiterations of the track-lock test, a TestFlag parameter which identifiesthe success or failure of the current test, and a TrackLock parameterwhich represents whether the target is currently being tracked. In someembodiments, processor 204 can set the value of the ScanFlag andScanMode parameter while MDA 203 can set the value of the TestFlag andTrackLock parameters as will be described below. This state diagram andthe parameters are exemplary and are only intended to represent one wayin which MDA 203 and processor 204 could be configured to interact.

Whenever the ScanFlag parameter is set to 0, monopulse antenna system200 will be in an idle state (i.e., neither scanning nor tracking).Whenever it is desired to commence the mainlobe detection process,processor 204 can set the ScanFlag parameter to 1 thereby transitioningmonopulse antenna system 200 into the scanning state. As part of thistransition, processor 204 can set the ScanMode parameter to 0 therebyindicating that open loop scanning should be performed (i.e.,instructing MDA 203 that it will not control the steering of antenna201). As open loop scanning is commenced, the MDA 203 can commenceperforming the initial power-level test while open loop scanning isoccurring.

Monopulse antenna system 200 will remain in the initial power-level teststate until either the initial power-level test succeeds or the scan iscompleted. As long as the ScanFlag parameter remains set, the system mayreset the scan and continue performing the scan pattern. In contrast, ifthe test succeeds, MDA 203 can set the TestFlag parameter to 1 therebycausing the system to transition into the track-lock test state. Inresponse to the TestFlag parameter being set to 1, processor 204 can setthe ScanMode parameter to 1 to indicate to MDA 203 that it shouldcommence closed loop tracking. Also, the Timeout and TestFlag parameterscan be reset so that they can be used during the track-lock test.

The track-lock test is performed in a similar manner as the initialpower-level test. However, each time the track-lock test is performed,the Timeout parameter can be incremented. If the Timeout parameterreaches a defined value x, the system will transition into the timeoutstate. If the ScanFlag parameter remains set, processor 204 can set theScanMode parameter to 0 to cause open loop scanning to be resumed andthe process restarted. In contrast, if the track-lock test succeeds, MDA203 can set the TestFlag parameter to 1 which will cause the system totransition into the tracking state. As part of this transition, theTestFlag parameter can again be reset.

Once in the tracking state, MDA 203 can commence tracking the objectusing one of the modes described above (magnitude-only or phase mode) oreven a hybrid tracking mode (for the purpose of antenna feed phasecorrection) which will be described below. While tracking, the systemwill repeatedly check the mainlobe-check test state. If themainlobe-check test succeeds, MDA 203 can set the TestFlag parameter to1 causing the system to remain in the tracking state and also set theTrackLock parameter to 1 to indicate to processor 204 that tracking isoccurring. In contrast, if the mainlobe-check test fails, the TestFlagparameter can be set to 0 to cause the system to transition to the lostlock state. The TrackLock parameter can also be set to 0 to indicatethat the object is no longer being tracked. When in the lost lock state,the system will return to open loop scanning as long as the ScanFlagparameter remains set. As part of this transition, processor 204 can setthe ScanMode parameter to 0 to inform that system that the MDA trackingcommands can be ignored.

The above described phase tracking is based on the premise that thephase transitions from positive to negative at the same location wherethe corresponding ratio is minimized. Hence, phase alignment to thephase response of the antenna comparator feed network is required forcorrect phase tracking. Based on this premise, by steering monopulseantenna 201 to these “phase zero crossings,” MDA 203 will also besteering monopulse antenna 201 to the point where the difference betweenthe sum channel power and the difference channel powers is maximized.However, there are situations where the phase zero crossings may notcoincide with the ratio nulls making phase tracking less effective oreven completely erroneous.

FIG. 6A provides a plot illustrating a case where the azimuth phase zerocrossing coincides with the azimuth ratio null. In contrast, FIG. 6Bprovides a plot illustrating a case where the azimuth phase zerocrossing is shifted to the right of the azimuth ratio null. Similarplots could be created for elevation but are not shown for sake ofsimplicity.

In these plots, the center horizontal line represents where the phase ofthe azimuth ratio transitions between positive and negative. Thepositive and negative values of the phase correspond directly to thesign of the azimuth or elevation ratio used to steer the antenna inphase tracking mode. In FIG. 6A, this phase zero crossing occurs at 0°azimuth which is also where the magnitude of the azimuth ratio isminimized. If it is assumed that the magnitude and phase of theelevation ratio similarly coincide, phase tracking would cause monopulseantenna 201 to be steered towards 0° azimuth and 0° elevation. In otherwords, the plot in FIG. 6A corresponds to what is shown in FIG. 5C.

In contrast, in FIG. 6B, the phase of the azimuth ratio is shifted tothe right. Specifically, the phase zero crossing is shown as occurringat approximately 0.8° azimuth where the magnitude of the azimuth ratiois not minimized. In this scenario, phase tracking would cause monopulseantenna 201 to be steered away from the null in the azimuth ratio. Moreparticularly, even if monopulse antenna 201 is initially orienteddirectly towards the azimuth null at 0°, the sign of the azimuth ratiowill be negative indicating that the object is positioned to the left ofthe bore axis. If in phase mode, MDA 203 would steer monopulse antenna201 to the left, away from the object. This leftward steering wouldcontinue until monopulse antenna 201 is offset by 0.8° to the left ofthe object. As shown in FIG. 6B, at this offset, the azimuth ratio isgreatly increased which would negatively impact tracking. A similarresult would happen if the phase of the elevation ratio is shifted fromthe elevation ratio null.

In a worst case scenario, the phase may be shifted so far that the phaseis reversed at the corresponding ratio null. FIG. 6C illustrates thiscase for the azimuth ratio. As shown, the phase is positive to the leftof the null and negative to the right of the null. Therefore, when phasetracking is employed, MDA 203 would steer monopulse antenna 201 awayfrom the null and outside the mainlobe mistakenly believing that it isactually steering it towards the null. In essence, when the phase isshifted too much, the mainlobe will appear as a sidelobe during phasetracking thereby causing open loop scanning to be resumed even thoughthe mainlobe had been located.

To correct for these phase shift scenarios, MDA 203 can be configured toperform “hybrid mode” tracking which employs a combination of both themagnitude-only tracking mode and the phase tracking mode of the azimuthand elevation ratios. In general, using phase mode for tracking istypically preferred to magnitude-only mode since it offers higheraccuracy, is faster, has increased bandwidth response, and greaterpull-in range. Hybrid mode tracking allows phase to be used even when amisalignment in the phase is occurring.

As an overview, in hybrid mode tracking, MDA 203 can initially performmagnitude-only tracking to steer monopulse antenna 201 towards theazimuth and elevation nulls. During this magnitude-only tracking, MDA203 can generate various thresholds that will later be employed duringphase tracking to ensure that a phase misalignment does not causemonopulse antenna 201 to be steered substantially away from the nulls.Then, MDA 203 can perform phase tracking including performing variouscalculations to correct (or offset) a phase misalignment. If, duringphase tracking, it is determined that monopulse antenna 201 has becomesubstantially misaligned, magnitude-only tracking can be resumed untilthe nulls are again located. This process of switching betweenmagnitude-only tracking and phase tracking can be repeated as necessaryto ensure that phase tracking can be accurately performed even when aphase misalignment is occurring.

Prior to describing hybrid mode tracking in detail, it is to beunderstood that, during either magnitude-only or phase tracking, MDA 203will constantly make slight adjustments to the boresight angle ofmonopulse antenna 201 (i.e., steer the antenna) in an effort to alignthe boresight angle with the target (which may be moving relative to theantenna). In magnitude-only tracking, MDA 203 will steer monopulseantenna 201 to attempt to minimize the azimuth and elevation ratioswithout using the phase response to determine the sign of the trackingerror, whereas, in phase tracking, MDA 203 will steer monopulse antenna201 to attempt to locate the phase zero crossings in the azimuth andelevation ratios. Accordingly, after each steering iteration, MDA 203will generate current azimuth and elevation ratios and then calculate,from these current ratios, how to subsequently steer monopulse antenna201.

FIGS. 7A and 7B provide a state diagram for hybrid mode tracking inaccordance with embodiments of the present invention. FIG. 7Aillustrates the portion of the state diagram representing magnitude-onlytracking, while FIG. 7B illustrates the portion of the state diagramrepresenting phase tracking. This state diagram can represent theprocess that is performed in the Tracking state shown in FIG. 4. Asindicated above, in hybrid mode tracking, magnitude-only tracking can beperformed to initially locate the nulls and then phase tracking canperform phase correction if needed.

Turning to FIG. 4, when the track-lock test passes, MDA 203 willcommence tracking in the tracking state. When hybrid-mode tracking isenabled, as shown in FIG. 7A, this can include transitioning from thetrack-lock test state to an initialize state in which a number oftracking parameters are initialized (as represented by MagTrackReset()). These tracking parameters include a MagCnt parameter which tracksthe number of magnitude tracking iterations that have been performed, anAzRatioMag parameter which stores a value representing the sum of theabsolute value of the magnitudes of the azimuth ratio within the MagCntperiod, an ElRatioMag parameter which stores a corresponding elevationvalue representing the sum of the absolute value of the magnitudes ofthe elevation ratios within the MagCnt period, an AzRatioDir parameterwhich stores a value representing a sum of the signs of the azimuthratio, and an ElRatioDir parameter which stores a value representing asum of the signs of the elevation ratio. Each of these parameters can beinitialized to zero (e.g., by calling MagTrackReset( )) as part oftransitioning into the magnitude-only tracking state.

As mentioned above, magnitude-only tracking entails calculating themagnitude of current values of the azimuth ratio (az_ratio) and theelevation ratio (el_ratio) and then steering monopulse antenna 201 tominimize the ratio. At each iteration of this steering (i.e., each timeMDA 203 causes the boresight angle of monopulse antenna 201 to bechanged), the absolute values of the azimuth and elevation ratios can beadded to the AzRatioMag and ElRatioMag parameters respectively.Therefore, at any given time, each of these two parameters will store asum of the magnitudes of the corresponding ratios that have beencalculated during the previous iterations of magnitude tracking. Thepurpose of this summation is to eventually compute an average ratiomagnitude value that can be used for future comparison. Also, at eachiteration of this steering, the AzRatioDir and ElRatioDir parameters canbe updated by adding 1 or −1 to the value of the parameter based on thesign of the current azimuth or elevation ratio respectively. Inparticular, when the corresponding ratio has a positive value (e.g. whenthe target is to the right of or above the boresight angle), theparameter can be incremented by 1, whereas when the corresponding ratiohas a negative value (e.g., when the target is to the left of or belowthe boresight angle), the parameter can be decremented by 1. In theory,for a perfect tracking system, the result of this calculation whentracking perfectly at the null would average out to 0. At eachiteration, a value of the MagCnt parameter can also be incremented by 1.

This loop can be repeated until a specified number of iterations havebeen performed. For example, the MagTrackHistory parameter can be set toa desired value (e.g., 250). Then, once the MagCnt parameter reaches thevalue of the MagTrackHistory parameter, the phase mode transition (PMT)test can be performed to determine whether the process should transitioninto phase tracking. Assuming the value of the MagTrackHistory parameteris 250, 250 iterations of magnitude tracking will be performed prior toperforming the PMT test. In this case, both the AzRatioMag andElRatioMag parameters would equal the sum of the magnitudes of the 250previous azimuth and elevation ratios, respectively. At this point, theywill each be divided by MagCnt to obtain the average ratio value for theazimuth and elevation channels. These parameters can be employed laterduring phase tracking as will be described below.

The PMT test can be employed to determine whether monopulse antenna 201has been steered sufficiently towards the azimuth and elevation nulls.For example, in some embodiments, the PMT test can determine whether theabsolute values of both the AzRatioDir and ElRatioDir parameters arebelow a defined threshold (e.g., IF ((Abs(AzRatioDir)<Threshold) and(Abs(ElRatioDir)<Threshold)). For example, when the MagTrackHistoryparameter is set to 250, the Threshold used in the PMT test can be 125.This Threshold can be a configurable parameter to allow monopulseantenna system 200 to be configured for a specific environment or for aspecific target. For example, faster moving targets may require a largerthreshold.

If the null has been found, MDA 203 would typically cause monopulseantenna 201 to dither about this null such that the sign of the azimuthand elevation ratios (using the magnitude-only mode tracking definitionof a sign-change) will periodically switch back and forth. Therefore,when the null has been found, the values of the AzRatioDir andElRatioDir should be minimized and the PMT test will pass causing phasetracking to be commenced. In contrast, if the PMT test fails, theprocess can be repeated by resetting the values of each of theparameters (e.g., by calling MagTrackReset( ) to set MagCnt, AzRatioMag,ElRatioMag, AzRatioDir, and ElRatioDir equal to 0) and returning to themagnitude tracking state as shown in FIG. 7A.

As an example of magnitude-only tracking, if the target is initially tothe right of the boresight angle, the azimuth ratio will have amagnitude value of Y. In this scenario, and having known the previousdirection, MDA 203 will steer monopulse antenna 201 slightly to theright, reducing the azimuth ratio eventually to a magnitude of Z, whichrepresents the minimum azimuth ratio magnitude achieved by the azimuthdifference channel null. This process will be repeated until the azimuthratio magnitude becomes greater than Z (i.e., once the target is to theleft of the boresight angle). Therefore, as monopulse antenna 201 issteered towards the null, the value of AzRatioDir will be positive for anumber of iterations. If the azimuth ratio is positive for 250consecutive iterations (meaning that the null has yet to be reached),the value of the AzRatioDir parameter will be 250 therefore causing ThePMT test to fail and magnitude tracking to be continued for 250 moreiterations. In contrast, if the azimuth ratio is positive for 100consecutive iterations (which would result in the AzRatioDir parameterreaching a value of 100) and then commences switching back and forthbetween increasingly larger magnitudes, the AzRatioDir would remainclose to 100 (or less) such that The PMT test would pass. In this way,the AzRatioDir and ElRatioDir parameters can be used to determine whenthe nulls have been found during magnitude-only mode and therefore thatphase tracking can be commenced.

Turning now to FIG. 7B, upon transitioning to phase tracking, a numberof parameters can be initialized for use in the phase tracking process.These parameters include a SgnCnt parameter which tracks the number ofphase tracking iterations that have been performed, a phase lock counter(PLC) parameter that is used to determine when phase lock has occurred,a sign history (SH) parameter which is used to define how manyiterations of phase tracking will be performed before a phase errorcalculation will be performed, a PhaseLock parameter which defineswhether phase lock has occurred, an AzRatioPhase parameter which storesa value representing the sum of the absolute value of the magnitudes ofthe azimuth ratios within a SgnCnt period, and an ElRatioPhase parameterwhich stores a value representing the sum of the absolute value of themagnitudes of the elevation ratios within a SgnCnt period. In thisexample, it is assumed that the SH parameter is initially set to 10while the other parameters are initially reset to 0.

As compared to the magnitude-only tracking method described above,during traditional phase tracking, MDA 203 will steer monopulse antenna201 based on the current phases (or signs) of the azimuth and elevationratios. Assuming there is no phase error, the phase response of thedifference channel signals should be zero at the nulls, and therefore,MDA 203 will steer monopulse antenna 201 to attempt to remain near thesephase zero crossings. This can be accomplished by steering the monopulseantenna by an azimuth step and an elevation step at each iteration. Thedirection of each step will be based on the sign of the correspondingazimuth and elevation tracking ratios. Therefore, MDA 203 will causemonopulse antenna 201 to dither about the phase zero crossings (i.e., itwould repeatedly steer the antenna in stepped increments in onedirection until the sign changes and then return in stepped incrementsin the opposite direction). However, if there is a phase error, thephase zero crossings will not align with the nulls which would result inthis dithering occurring away from the nulls, or, if the phase error issignificant, in the eventual failure of the mainlobe-check test and thereturn to open loop scanning.

To account for these phase error scenarios, the phase error correctionprocess depicted in FIG. 7B can be implemented. Prior to describing thephase error correction process, it is to be understood that the phaseerror that is to be corrected exists in the signal received at theindividual antenna elements as opposed to phase drift that may occur oneach individual channel as the received signals are processed withinmonopulse antenna system 200, or even the sum-to-difference channelphase alignment to compute the difference channel ratio. The phase driftthat can occur on each individual channel may be due to different phaseerrors that are introduced on a component-by-component basis during theindependent processing of the channels (e.g., an LNA used to produce thesum channel may introduce a different phase error than an LNA used toproduce the azimuth difference channel). Such phase drift is alsoexacerbated over frequency and temperature. This phase error can beovercome by some amount of hardware unit calibration and is notdiscussed in this patent. Furthermore, phase alignment is essential tocomputing azimuth and elevation tracking ratios. The crucial phasealignment in this case is the sum channel signal phase relative to thedifference channel signal phase. Techniques exist for accounting forthis type of phase alignment when digitally combining the sum anddifference channels (see e.g., U.S. patent application Ser. No.14/572,470) but such techniques are not the subject of the presentinvention. However, the phase error correction process of the presentinvention can be used in conjunction with such phase drift calibrationor sum-to-difference channel phase alignment/correction techniques. Inshort, the present invention can allow phase tracking to be employedeven when the signals received at the individual antenna elements arenot phase aligned with the down-stream RF and digital processing.

To enable phase errors to be corrected, monopulse antenna system 200 caninclude a phase shifter 801 and 802 in the azimuth difference channeland elevation difference channel respectively as shown in FIG. 8. Phaseshifters 801 and 802 can preferably be implemented on an FPGA that formspart of MDA 203. Therefore, even though for illustrative purposes FIG. 8depicts phase shifters 801 and 802 as separate from MDA 203 (which isone possible way in which the phase shifters 801 and 802 could beimplemented), for speed and efficiency reasons, FPGA-implemented phaseshifters may typically be employed. The process depicted in FIG. 7B willcalculate a phase shift that should be applied by each of phase shifters801, 802 to account for a phase error. MDA 203 can calculate therequired phase shift in a systematic method until the current phaseshift corrects for all measured phase error. MDA 203 can perform thisprocess independently for each difference channel as will now bedescribed in detail.

As shown in FIG. 7B, after the conditions of the PMT test are met, MDA203 will transition into phase tracking. As part of this transition, theSH parameter can be set to an initial value (e.g., 10) while the PLC,PhaseLock, AzRatioPhase, and ElRatioPhase parameters can be set to 0. Asindicated above, during phase tracking, MDA 203 will steer monopulseantenna 201 in azimuth and elevation steps based on the phase (or sign)of the current azimuth and elevation ratios. Initially, it can beassumed that no phase error exists (i.e., that the phase zero crossingsalign with the nulls in the ratios) and therefore that phase shifters801, 802 are configured to apply no phase shift (i.e., a 0° shift) tothe corresponding difference channel.

As shown, at each iteration of phase tracking, MDA 203 can add theabsolute value of the azimuth and elevation ratios to the AzRatioPhaseand ElRatioPhase parameters respectively. Also, at each iteration, MDA203 will compare the current magnitude of the azimuth and elevationratios to a Radius Threshold (RT) parameter. The RT parameter can be aconfigurable parameter that controls when MDA 203 will return tomagnitude-only tracking. For example, RT can be set to 0.25 (assumingthe ratios during magnitude-only tracking were lower than this amount)such that, whenever either the azimuth or elevation ratio exceeds 0.25(which would indicate that monopulse antenna 201 is at potential riskfor failing the mainlobe-check test), MDA 203 will resume magnitude-onlytracking to return to tracking near the difference channel nulls.

As long as the azimuth and elevation ratios remain below RT, MDA 203will continue to sum the magnitudes of the ratios until SgnCnt (which isincremented with each iteration) equals SH. As mentioned above, SH caninitially be set to 10 such that 10 iterations will initially beperformed. After these 10 iterations, the null-alignment test (NAT) willbe evaluated based on the average values of the AzRatioPhase andElRatioPhase parameters (based on SgnCnt iterations) and the storedvalues of the AzRatioMag and ElRatioMag parameters. In other words, MDA203 can compare the magnitudes of the ratios during phase tracking tothe magnitudes of the ratios during the previous magnitude tracking todetermine whether the current magnitudes have increased beyond athreshold.

For example, the NAT can be: IF AzRatioPhase/SH<(AzRatioMag/MTH) and IFElRatioPhase/SH<(ElRatioMag/MTH). The NAT can therefore determinewhether the average magnitude of the ratios during SH iterations ofphase tracking is less than the average magnitude of the ratios duringthe MTH iterations of magnitude tracking that were performed prior tocommencing phase tracking. In some embodiments, the average magnitude ofthe magnitude-only mode ratios can be scaled by a factor of X (e.g., IFAzRatioPhase/SH<(AzRatioMag/MTH)*X and IFElRatioPhase/SH<(ElRatioMag/MTH)*X. As an example, X can be set to 1.25.The primary purpose of the NAT is to determine whether phase trackinghas kept monopulse antenna 201 oriented towards the nulls. Since theaverage ratios from magnitude-only tracking should represent the averageratios at the nulls, the NAT will identify whether phase tracking causedmonopulse antenna 201 to be steered away from the nulls. Scaling by Xcan prevent the NAT from failing when there are only slight differencesbetween the average magnitude and phase ratios.

If the NAT passes, it is assumed that monopulse antenna 201 is stillnear the nulls and a determination of whether a phase lock has alreadybeen established can be performed. In contrast, if the NAT fails, it isassumed that monopulse antenna 201 has been steered away from thenull(s) due to the phase zero crossing(s) not aligning with the null(s).If the NAT fails, a phase correction can be calculated and applied tothe appropriate difference channels. It is noted that a phase error mayexist only in one ratio. Therefore, MDA 203 can perform the phase errorcorrection process independently on each ratio. In particular, the NATmay be performed independently on each ratio leading to an independentcalculation of a phase correction for each difference channel. However,for ease of illustration, it will be assumed that a similar phase errorexists in both difference channels such that the phase error correctionprocess will proceed in sync for both ratios.

Assuming the NAT passes for both ratios, MDA 203 will proceed todetermine whether a phase lock has been established. Each time the NATpasses, PLC can be incremented by 1 and SH can be incremented by 50 (orsome other reasonable value) unless SH has already reached 1000 (or someother reasonable value) in which case SH will remain at 1000. Then, aphase lock determination can be made. In this case, if PLC is greaterthan 10 and SH equals 1000, it can be determined that phase lock hasoccurred. Therefore, for phase lock to occur, the average magnitudes ofthe ratios must remain below the average magnitudes (or scaled averagemagnitudes) of the ratios that existed during magnitude-only trackingfor a large number of iterations. This would be the case if phasetracking is keeping monopulse antenna 201 at the nulls. If phase lockhas occurred, the PhaseLock parameter can be set to 1 and phase trackingcan be continued without any adjustment to the current phase corrections(i.e., without updating phase shifters 801, 802).

In contrast, if phase lock has not yet occurred, updated phasecorrections can be calculated at phase adjustment block 701 a. Thespecific manner in which an updated phase correction can be calculatedwill be described below. Because the NAT has passed thereby implyingthat monopulse 201 is still at or near the nulls, a relatively smalladjustment can be made to the phase corrections. In other words, it canbe assumed that the current phase corrections are substantiallyoffsetting any phase error that may exist but that it may be possible toimprove the phase corrections. MDA 203 can therefore continue to makethese slight phase adjustments in an attempt to completely offset anymeasurable phase error.

Also, the direction of this phase adjustment can be based on whether themagnitude of the corresponding ratio is increasing or decreasing. Inother words, if the magnitude of AzRatioPhase (using azimuth as anexample) computed at the SH interval is larger than the AzRatioPhaseduring the previous SH period, it can be assumed that the current phasecorrection does not perfectly align the phase zero crossing with thenull and therefore the phase correction should be adjusted in a negativedirection (i.e., leftward or downward with respect to the exampleorientation used above). In practice, phase adjustment block 701 a willmake repeated small adjustments to the phase correction in one directionuntil the phase correction causes the phase zero crossing to bepositioned on the opposite side of the null (from the perspective of MDA203). At this point, phase adjustment block 701 a will make repeatedadjustments in the other direction. Assuming monopulse antenna 201remains in the nulls, this process will continue until phase lock isestablished.

Whenever the NAT fails, the process will flow to either phase adjustmentblock 701 b if phase lock has been established (i.e., if the PhaseLockparameter is set to 1) or to phase adjustment block 701 c if phase lockhas not been established. In each of phase adjustment blocks 701 b, 701c, updated phase corrections can be calculated. With regards to phaseadjustment block 701 b, because phase lock had been established, theadjustment to the phase correction can be small but slightly greaterthan the adjustment made in phase adjustment block 701 a, whereas, withregard to phase adjustment block 701 c, because phase lock has not beenestablished, a relatively large adjustment can be made. Again, theseadjustments to the phase corrections can be made to attempt to moreaccurately offset the actual phase error. The primary role of the phaseerror correction process therefore is to repeatedly increment the phasecorrections until the measurable phase error is minimized. Whenever theNAT fails, SH can be set to 15 (or some other reasonable value) andSgnCnt can be reset to cause 15 iterations to be performed prior toagain evaluating the NAT.

Finally, if either of the ratios exceeds RT, MDA 203 can transition backto magnitude-only tracking including calculating new phase correctionsat phase adjustment block 701 d. In this case, it can be assumed thatthe current phase corrections are not accurate and should besubstantially updated. Therefore, phase adjustment block 701 d can applya large (e.g., 90°) adjustment to the phase corrections prior toresuming magnitude tracking. The adjustments made by phase adjustmentblock 701 d will remain during magnitude-only tracking and until asubsequent adjustment is made once phase tracking is again resumed. Oncephase tracking is resumed, the same process will be performed to againlocate a phase correction that will offset the phase error.

If the phase error correction process reaches phase adjustment blocks701 a or 701 b, it can be assumed that the current phase corrections arerelatively accurate. Therefore, phase adjustment blocks 701 a, 701 b canbe configured to make small adjustments to the phase corrections. FIGS.9A and 9B generally illustrate how this can be done for the azimuthdifference channel. In FIG. 9A, it is assumed that phase shifter 801 iscurrently configured to apply a phase shift of 10° to the azimuthdifference channel. While this 10° phase shift is being applied, MDA 203can perform the phase error correction process including calculating themagnitude of the azimuth ratio at each steering iteration. Asrepresented by StoreCurrRatios( ) in FIG. 7B, MDA 203 can be configuredto retain the values of the previous AzRatioPhase and ElRatioPhasecomputed during the previous SH loop period (e.g., as values ofPrevAzRatioPhase and PrevElRatioPhase parameters) to allow MDA 203 todetermine whether the ratios are increasing or decreasing in subsequentperiods. In other words, the phase corrections calculated in phaseadjustments blocks 701 a and 701 b are based on a comparison of thecurrent ratio average to the corresponding previous ratio average. Inthis example, it will be assumed that the current AzRatioPhase (computedafter reaching SH SgnCnt iterations) averages to a magnitude of 0.14,but the prevAzRatioPhase magnitude during the previous SH period had anaverage of 0.12, thereby indicating that monopulse antenna 201 iscurrently being steered away from the azimuth null. Therefore, thecomputed phase adjustment block 701 a can determine that the phasecorrection being applied by phase shifter 801 should be reduced.

Assuming that phase adjustment block 701 a is configured to makeapproximately a 2.8° adjustment (represented by 8 steps of a 10-bitdigital phase-shifter), MDA 203 can apply this adjustment as −2.8° tothe current phase shift (i.e., 8 steps or (π/64)*NewDirection, whereNewDirection is + when the ratio is decreasing and − when the ratio isincreasing). Accordingly, FIG. 9B shows the result of the exampledescribed in FIG. 9A, where MDA 203 outputs a signal to phase shifter801 causing phase shifter 801 to apply a phase shift of 7.2°. A similarcalculation, but with a different phase step size will be performed byphase adjustment block 701 b. For example, phase adjustment block 701 bcould be configured to apply an 11.25° adjustment to the current phasecorrection where this adjustment will be added to or subtracted from thecurrent phase correction depending on whether the azimuth ratio isdecreasing or increasing respectively (i.e., 32 steps or(π/16)*NewDirection). This calculation of the adjustment to the phasecorrection can be performed independently for each of the azimuth andelevation ratios so that a proper adjustment can be supplied to each ofphase shifters 801, 802. In short, phase adjustment blocks 701 a, 701 bcan both be configured to apply a fixed adjustment to the current phaseshift in a direction that is dependent on whether the correspondingratio is increasing or decreasing.

Phase adjustment blocks 701 c, 701 d can be configured to calculate anadjustment in a slightly different manner. Rather than employingprevious ratio magnitude averages to determine whether a ratio isincreasing or decreasing, each of phase adjustment blocks 701 c, 701 dcan employ the sin( ) of the current ratio to determine the direction ofthe adjustment as well as to scale the amount of the adjustment. Forexample, phase adjustment block 701 c can calculate the adjustment as 64steps of a 10-bit digital phase shifter (π/8)*sin(ratio) and phaseadjustment block 701 d can calculate the adjustment as 256 steps of a10-bit phase shifter (π/2)*sin(ratio).

In summary, phase adjustment blocks 701 a-701 c can each be configuredto calculate an adjustment to the phase corrections that are currentlybeing applied by phase shifters 801, 802 in an attempt to offset anyphase error that may exist during subsequent phase tracking. Incontrast, phase adjustment block 701 d can apply a large adjustment tothe phase corrections prior to transitioning back to magnitude trackingsince these current phase corrections will have caused phase tracking toincorrectly steer monopulse antenna 201 away from the nulls.

When a phase error exists, phase tracking will initially steer monopulseantenna 201 away from the null. As this steering is occurring, MDA 203will detect that the ratios are increasing when the NAT fails and canthen adjust the phase correction being applied by phase shifter 801and/or 802. These repeated adjustments to the phase correction shouldultimately offset the phase error thereby aligning the phase zerocrossings with the nulls. Once this alignment occurs, phase lock will beestablished and the NAT will be applied on a less frequent interval.Absent any significant change to the phase error, this phase lock willbe retained thereby allowing MDA 203 to perform phase trackingthroughout the tracking process. In other words, once phase lock isobtained, no more phase corrections are required unless there is sometracking event that causes the tracking to revert back to magnitude-onlytracking mode.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description.

What is claimed:
 1. A method for performing tracking in a monopulse antenna system that provides tracking in one of an azimuth axis or an elevation axis or in both the azimuth axis and the elevation axis, the method comprising: performing magnitude-only tracking; during magnitude-only tracking, storing one or both of a first azimuth magnitude parameter that represents a magnitude of the azimuth ratio over a number of magnitude steering iterations or a first elevation magnitude parameter that represents a magnitude of the elevation ratio over the number of magnitude steering iterations, and also storing one or both of an azimuth direction indicator that represents a phase of the azimuth ratio over the number of magnitude steering iterations or an elevation direction indicator that represents a phase of the elevation ratio over the number of magnitude steering iterations; after magnitude-only tracking has been performed for the number of magnitude steering iterations, comparing one or both of the azimuth direction indicator or the elevation direction indicator to a defined threshold; and upon determining that one or both of the azimuth direction indicator or the elevation direction indicator is below the defined threshold, commencing phase tracking.
 2. The method of claim 1, further comprising: during phase tracking, determining whether one or both of a current magnitude of the azimuth ratio or a current magnitude of the elevation ratio exceeds a defined threshold; and when either the azimuth ratio or the elevation ratio exceeds the defined threshold, recommencing magnitude-only tracking.
 3. The method of claim 2, wherein the monopulse antenna system includes one or both of an azimuth phase shifter for applying a phase correction to an azimuth difference channel or an elevation phase shifter for applying a phase correction to an elevation difference channel, the method further comprising: in conjunction with recommencing magnitude-only tracking, modifying the phase correction being applied by one or both of the azimuth phase shifter or the elevation phase shifter.
 4. The method of claim 3, wherein the phase correction is modified based on a current magnitude and phase of the corresponding ratio.
 5. The method of claim 2, further comprising: during phase tracking, storing one or both of a second azimuth magnitude parameter that represents the magnitude of the azimuth ratio over a number of phase steering iterations or a second elevation magnitude parameter that represents the magnitude of the elevation ratio over the number of phase steering iterations; and after phase tracking has been performed for the number of phase steering iterations, determining whether the second azimuth magnitude parameter is less than the first azimuth magnitude parameter or determining whether the second elevation magnitude parameter is less than the first elevation magnitude parameter.
 6. The method of claim 5, wherein one or both of the second azimuth magnitude parameter or the second elevation magnitude parameter comprises an average of the magnitude of the corresponding ratio over the number of phase steering iterations, and one or both of the first azimuth magnitude parameter or the first elevation magnitude parameter comprises an average of the magnitude of the corresponding ratio over the number of magnitude steering iterations.
 7. The method of claim 6, wherein one or both of the first azimuth magnitude parameter or the first elevation magnitude parameter is scaled by a specified amount.
 8. The method of claim 5, wherein the monopulse antenna system includes one or both of an azimuth phase shifter for applying a phase correction to an azimuth difference channel or an elevation phase shifter for applying a phase correction to an elevation difference channel, the method further comprising: when it is determined that the second azimuth magnitude parameter is less than the first azimuth magnitude parameter or that the second elevation magnitude parameter is less than the first elevation magnitude parameter, determining whether phase lock has occurred such that: when phase lock has occurred, phase tracking is continued without modifying the phase correction being applied by one or both of the azimuth phase shifter or the elevation phase shifter; whereas when phase lock has not occurred, phase tracking is continued after modifying one or both of the phase correction being applied by the azimuth phase shifter or the elevation phase shifter.
 9. The method of claim 8, wherein the phase correction being applied by one or both of the azimuth phase shifter or elevation phase shifter is modified by a set amount in a direction that is determined based on whether the corresponding ratio has been increasing or decreasing during previous phase steering iterations.
 10. The method of claim 8, wherein determining whether phase lock has occurred comprises determining whether the second azimuth magnitude parameter has remained less than the first azimuth magnitude parameter or that the second elevation magnitude parameter has remained less than the first elevation magnitude parameter for a specified number of phase steering iterations.
 11. The method of claim 5, wherein the monopulse antenna system includes one or both of an azimuth phase shifter for applying a phase correction to an azimuth difference channel or an elevation phase shifter for applying a phase correction to an elevation difference channel, the method further comprising: when it is determined that the second azimuth magnitude parameter is not less than the first azimuth magnitude parameter or that the second elevation magnitude parameter is not less than the first elevation magnitude parameter, continuing phase tracking after modifying the phase correction being applied by one or both of the azimuth phase shifter or the elevation phase shifter.
 12. The method of claim 11, wherein modifying the phase correction being applied by one or both of the azimuth phase shifter or the elevation phase shifter comprises applying a first adjustment amount or a second adjustment amount.
 13. The method of claim 12, wherein the first adjustment amount is applied when phase lock has occurred and the second adjustment amount is applied when phase lock has not occurred.
 14. The method of claim 12, wherein the first adjustment amount is a set amount in a direction that is determined based on whether the corresponding ratio has been increasing or decreasing during previous phase steering iterations, and the second adjustment amount is determined based on a current magnitude and phase of the corresponding ratio.
 15. A monopulse antenna system comprising: a monopulse antenna comprising a number of monopulse antenna elements; a comparator network that generates a sum channel, an azimuth difference channel, and an elevation difference channel from a signal received at the monopulse antenna; and a monopulse detector assembly that receives the sum channel, the azimuth difference channel, and the elevation difference channel from the comparator network and generates an azimuth ratio and an elevation ratio from the channels; the monopulse detector assembly being configured to perform hybrid tracking by: while performing magnitude-only tracking, storing a first azimuth magnitude parameter that represents a magnitude of the azimuth ratio over a number of magnitude steering iterations and a first elevation magnitude parameter that represents a magnitude of the elevation ratio over the number of magnitude steering iterations, and also storing an azimuth direction indicator that represents a phase of the azimuth ratio over the number of magnitude steering iterations and an elevation direction indicator that represents a phase of the elevation ratio over the number of magnitude steering iterations; after magnitude-only tracking has been performed for the number of magnitude steering iterations, comparing the azimuth direction indicator and the elevation direction indicator to a defined threshold; and upon determining that the azimuth direction indicator and the elevation direction indicator are below the defined threshold, commencing phase tracking.
 16. The monopulse antenna system of claim 15, further comprising: an azimuth phase shifter that is configured to apply a phase correction to the azimuth difference channel before the monopulse detector assembly generates the azimuth ratio; and an elevation phase shifter that is configured to apply a phase correction to the elevation difference channel before the monopulse detector assembly generates the elevation ratio.
 17. The monopulse antenna system of claim 16, wherein the monopulse detector assembly is further configured to: store, during phase tracking, a second azimuth magnitude parameter that represents a magnitude of the azimuth ratio over a number of phase steering iterations and a second elevation magnitude parameter that represents a magnitude of the elevation ratio over the number of phase steering iterations; and after phase tracking has been performed for the number of phase steering iterations, determine whether the second azimuth magnitude parameter is less than the first azimuth magnitude parameter and determining whether the second elevation magnitude parameter is less than the first elevation magnitude parameter.
 18. The monopulse antenna system of claim 16, wherein the monopulse detector assembly is further configured to: adjust the phase correction being applied by the azimuth and elevation phase shifters based on the determination of whether the second azimuth magnitude parameter is less than the first azimuth magnitude parameter and on the determination of whether the second elevation magnitude parameter is less than the first elevation magnitude parameter.
 19. A method for detecting the mainlobe in a monopulse antenna system and then tracking on the mainlobe, the method comprising: during open loop scanning, performing an initial power-level test to identify when a lobe has been located; in response to the initial power-level test passing, commencing closed loop scanning; during closed loop scanning, performing a track-lock test to identify when the mainlobe has been located; and in response to the track-lock test passing, commencing hybrid tracking in which magnitude-only tracking is initially performed and then phase tracking is performed, the hybrid tracking comprising: commencing magnitude-only tracking; during magnitude-only tracking, storing a first azimuth magnitude parameter that represents a magnitude of the azimuth ratio over a number of magnitude steering iterations and a first elevation magnitude parameter that represents a magnitude of the elevation ratio over the number of magnitude steering iterations, and also storing an azimuth direction indicator that represents a phase of the azimuth ratio over the number of magnitude steering iterations and an elevation direction indicator that represents a phase of the elevation ratio over the number of magnitude steering iterations; after magnitude-only tracking has been performed for the number of magnitude steering iterations, comparing the azimuth phase parameter and the elevation phase parameter to a defined threshold; and upon determining that the azimuth direction indicator and the elevation direction indicator are below the defined threshold, commencing phase tracking.
 20. The method of claim 19, wherein the initial power-level test comprises determining whether a sum channel power exceeds each of an azimuth difference channel power and an elevation difference channel power, and in addition comprises determining whether an azimuth ratio and an elevation ratio are both below an initial power-level threshold.
 21. The method of claim 19, wherein hybrid tracking further comprises: during phase tracking, storing a second azimuth magnitude parameter that represents a magnitude of the azimuth ratio over a number of phase steering iterations and a second elevation magnitude parameter that represents a magnitude of the elevation ratio over the number of phase steering iterations; and after phase tracking has been performed for the number of phase steering iterations, determining whether the second azimuth magnitude parameter is less than the first azimuth magnitude parameter and determining whether the second elevation magnitude parameter is less than the first elevation magnitude parameter. 